Black Tornado of Death
While the challenge only came out of Satoshi's mouth by accident, Black Fog took it as an insult and was severely offended by it. Up until recently, he's never heard the name of the kid. And he underestimated him, thinking that there was no chance of him winning against his superb beast taming skill.
Turns out, he couldn't hold a candle against him. Satoshi's minimal power was far superior to his, and there was no denying that fact.
Regardless, he took Satoshi's challenge as an offense against him. In front of so many people, Satoshi was making fun of him despite it not being his intention at all. Gritting his teeth, Black Fog decided to hit Satoshi with everything he's got.